Flow meters are usually employed for measuring the flow of a liquid material in a pipe. Prior art meters are of several types and they are typically based on such physical phenomena as the Coriolis force, pressure difference and voltage induced by the movement of a conductive material in the magnetic field. Furthermore, ultrasound techniques have been used to implement meters based on correlation and the Doppler effect. Microwave techniques have also been used to implement flow meters based on the Doppler effect. Solutions utilizing microwave correlation by using a metallic process pipe are also known, e.g. U.S. Pat. Nos. 4,423,623, 4,888,547 and WO patent publication 94/17373. In the solutions according to U.S. Pat. Nos. 4,423,623 and 4,888,547, the process pipe is used as a wave guide and the variations in the cut-off frequency of the wave guide act as correlating signals. In the solution of WO patent publication 94/17373, the correlation of signals at the same frequency or at least in the same frequency band is utilized after the signals have passed through a flowing material.
These techniques involve a plurality of drawbacks. For example, the problem in ultrasound measurement is that it is difficult to make ultrasound move from the transmitter sensor to the dielectric material of the tube wall and further to the material flowing inside the tube. However, it is particularly difficult to apply the above methods to pneumatic pipelines, since a magnetic flow meter, for instance, requires an adequately high electric conductivity of the material to be measured; a property which materials flowing in pneumatic transfer lines usually lack. Prior art solutions based on microwave techniques typically (U.S. Pat. Nos. 4,423,623 and 4,888,547) do not allow measurement of e.g. the flow of pulp in dielectric conduits, such as a rubber tube. In the solution according to WO patent publication 94/17373, in turn, the signals of different transmitter antennae interfere with each other upon receiving.